To an increasing extent, touch-sensitive panels are being used for providing input data to computers, electronic measurement and test equipment, gaming devices, etc. The panel may be provided with a graphical user interface (GUI) for a user to interact with using e.g. a pointer, stylus or one or more fingers. The GUI may be fixed or dynamic. A fixed GUI may e.g. be in the form of printed matter placed over, under or inside the panel. A dynamic GUI can be provided by a display screen integrated with, or placed underneath, the panel or by an image being projected onto the panel by a projector.
There are numerous known techniques for providing touch sensitivity to the panel, e.g. by using cameras to capture light scattered off the point(s) of touch on the panel, by using cameras to directly observe the objects interacting with the panel, by incorporating resistive wire grids, capacitive sensors, strain gauges, etc. into the panel.
In one category of touch-sensitive panels known as ‘above surface optical touch systems’ and known from e.g. U.S. Pat. No. 4,459,476, a plurality of optical emitters and optical receivers are arranged around the periphery of a touch surface to create a grid of intersecting light paths above the touch surface. Each light path extends between a respective emitter/receiver pair. An object that touches the touch surface will block or attenuate some of the light paths. Based on the identity of the receivers detecting a blocked light path, a processor can determine the location of the intercept between the blocked light paths.
US patent publication 2004/0252091 discloses an alternative technique which is based on frustrated total internal reflection (FTIR). Light is coupled into a panel to propagate inside the panel by total internal reflection. Arrays of light sensors are located around the perimeter of the panel to detect the light. When an object comes into contact with a surface of the panel, the light will be locally attenuated at the point of touch. The location of the object is determined by triangulation based on the attenuation of the light from each source at the array of light sensors.
For most touch systems, a user may place a finger onto the surface of a touch panel in order to register a touch. Alternatively, a stylus may be used. A stylus is typically a pen shaped object with one end configured to be pressed against the surface of the touch panel. An example of a stylus according to the prior art is shown in FIG. 3. Use of a stylus 60 may provide improved selection accuracy and pointer precision over a simple finger touch. This can be due to the engineered stylus tip 160 providing a smaller and/or more regular contact surface with the touch panel than is possible with a human finger. Also, muscular control of an entire hand in a pen holding position can be more precise than a single finger for the purposes of pointer control due to lifelong training in the use of pens and pencils.
Two types of stylus exist for touch systems. An active stylus is a stylus typically comprising some form of power source and electronics to transmit a signal to the host touch system. The type of signal transmitted can vary but may include position information, pressure information, tilt information, stylus ID, stylus type, ink colour etc. The source of power for an active stylus may include a battery, capacitor, or an electrical field for providing power via inductive coupling. Without power, an active stylus may lose some or all of its functionality.
An active stylus may be readily identified by a host system by receiving an electronic stylus ID from the active stylus and associating the stylus ID with position information relating to the contact position between the stylus and the touch surface of the host system.
A passive stylus has no power source and does not actively communicate with the host system. Therefore, a passive stylus is cheaper to manufacture than an active stylus and does not require maintenance. However, advanced information like application pressure, tilt information, stylus ID, stylus type, ink colour etc. can be significantly more difficult to obtain from a passive stylus than from an active stylus.
U.S. Pat. No. 6,567,078 describes a method of marking a plurality of passive styluses with one or more colour films in a pattern unique to each stylus. A camera is arranged to record the colour markings on the stylus and identify the passive stylus in use in order to determine the appropriate choice of ink colour to be displayed on the screen.
For optical touch systems such as those described in US patent publication 2004/0252091 and U.S. Pat. No. 4,459,476, it can be difficult to identify an object with a tip as small as a stylus. In particular, stylus tips are typically small (i.e. smaller than 4 mm in diameter) and provide a relatively small amount of attenuation of the light signals compared with a finger or other large object. The stylus tip may also have a smaller diameter than the resolution of the touch system is able to resolve.
Furthermore, the low signal-to-noise of such systems makes identification of each of a plurality of passive styluses using unique retro-reflective material arrangements difficult and unreliable.
Therefore, what is needed is a way of identifying objects touching an optical touch system which does not suffer from the above problem.